[nSLUG] D-Link router and other comments

Jim Haliburton jim at on-site.ns.ca
Fri Feb 5 13:29:57 AST 2010

Mike was having problems with his D-Link router.

If you sometime do not get a link with auto, it can be a case where each side 
is constantly round-robin ing.  They each cycle back and forth with a timing 
conflict during auto-negotiation, and you get no where.  Set the PC side at 
fixed 100, half as a starter.  Then see if you can connect.  Use the test 
software to confirm your card is set at 100 half fixed.  Some Linux drivers 
will NOT set the speed correctly if the wire is connected.  Some drivers do NOT 
reset the card correctly.  There are a slew of Intel cards that do not have 
drivers that reset correctly except at power off and plug removed from the 
power supply.  With these the only choice is to start the PC with the network 
cable disconnected from the PC.  Once all the drivers are loaded insert the 
cable.  This happens in other OSs besides Linux/Unix.

Some D-Link routers do not support auto polarity.  In those cases you may need 
a crossover cable rather than a straight through.

Warren R asserted that one should check the cables.  This is the predominant 
cause of poor connectivity.  I have a Fluke DSP-100 that shows clients that 
problem all the time.  However he also asserted that 100mb requires all 4 pair. 
 This is wrong.  

Gigabit requires all 4 pairs.  100 mb only requires pair sets 1-2, and  3-6 
just like 10 mb. 100 megabit will fail if one "splits the pairs".  In many self 
made cables the unknowing simply wire pair pair pair pair across the RJ-45.  
Doing this causes wire pair 3-6 to be from separate twisted pairs.  This will 
work very poorly if at all.  It is wrong.  The 3-6 pair set has to be from the 
same twisted pair set in the cable.  Simple test is to replace with a known 
good cable.  If it works with the new cable, cut the ends off the bad cable and 
throw it in the trash.  Keeping it will lead to having the same problem again 
when someone re-uses it.

10 mb will run on 2 pair  phone wire for short distances.  Twisted 2 pair is 
also sometimes called Cat 3, which is what 10 mb requires at a minimum.

An example of race condition in a network autonegotiation is with the windows 
document that asserts that Windows XP does not support back to back network 
connections between two computers without a switch.  This is because 
autonegotiation will never agree on a speed to lock on to.  The document has a 
shortcoming in that it fails to continue by saying that if a crossover cable is 
used and the cards are set explicitly at the same speed and duplex, preferably 
half, they will work just fine.  I have migrated gigabits of data from old to 
new stand alone PCs using this technique without a switch in between, on 
numerous times with many variations in NICs.

So in summary.

1)  Find if the router supports autopolarity.  If not use a crossover cable.
2)  Use a known good cable beware of "split pairs"
3)  Set explicit speed on the PC at half and reboot with cable out of PC.
4)  Change speed and reboot again
5)  Confirm that changing driver settings really changes card.
6)  (with tongue in cheek)  Buy a new cheap router. It will probably solve your 
problems for less that $30.00

Jim H

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